Large over-the-road trucks often include sleeping quarters in the cab of the truck. These sleeping quarters allow a driver to stay in the cab during mandatory rest periods. To maintain a comfortable environment for the driver, the cabs include heating and air conditioning systems. These systems are powered by the large diesel engines of the truck, requiring that the driver idle the engine while sleeping in the cab. The idling diesel engine, in addition to exhaust, produces much more power than is required by the heating or air conditioning system. As an alternative, an auxiliary engine, such as an engine used for cooling a refrigerated trailer, can be used to power the heating and/or air conditioning system, eliminating the need to idle the truck's diesel engine. Further, the auxiliary engine can drive an alternator which charges the batteries of the truck.
During operation of a cab air conditioning system, when the temperature of the cab rises above a threshold, the air conditioning system is engaged to cool the cab. When the temperature of the cab drops below a second threshold, the air conditioning system is disengaged. The auxiliary engine is typically capable of producing enough power to power all of its attached components (e.g., cab air conditioner, alternator) simultaneously. Because several of the components are often times not actively powered (e.g., when the cab air conditioner is disengaged), the auxiliary engine is oversized for some of the time, and thus uses more fuel than is needed to power the operating components.